In recent years, a display device has been flat-surface display device from a display using a cathode-ray tube toward a flat-surface display device called flat panel display (FPD) such as a liquid crystal panel and a plasma display. In the liquid crystal panel, as an element relating to pixel switching by liquid crystals, a thin-film transistor made of a—Si (amorphous silicon) or poly-silicon is used for a switching element. Recently, an FPD using organic Electro-Luminescence (EL) has been expected aiming for further larger area size and flexibility. However, since the organic EL display is a spontaneous light emitting device in which an organic semiconductor layer is driven to obtain direct light emission, a property of a current-driving device is required for a thin-film transistor which is different from conventional liquid crystal displays. Meanwhile, it is also required to provide new functions such as further larger area size and flexibility for a future FPD, and therefore, it is required to provide not only high performance as an image display device but also compatibility to a large-area process and compatibility to a flexible substrate. Based on such a background, using a transparent oxide semiconductor whose band gap is about 3 eV has been studied for the thin-film transistor for the display device in recent years, and usages for a thin film memory, an RFID, a touch panel, and others in addition to the display device have also been expected.
While zinc oxide or tin oxide has been known as the transparent oxide transistor for a long time, a thin-film transistor using IGZO (complex oxide composed of indium oxide, gallium oxide, and zinc oxide) is disclosed, IGZO being used as a material capable of suppressing threshold potential shift which is a bad point of the zinc oxide, in Japanese Patent Application Laid-Open Publication No. 2006-165532 (Patent Document 1), paragraphs [0009] to [0052] and Japanese Patent Application Laid-Open Publication No. 2006-173580 (Patent Document 2), paragraphs [0009] to [0032] (see Patent Documents 1 and 2), and a possibility of achieving a new semiconductor device by a thin-film process has been expected in recent years. More particularly, subthreshold swings of IGZO and ZTO (complex oxide composed of zinc oxide and tin oxide) better than that of poly-silicon have been confirmed, and it is considered that they are used to not only display application but also device application requiring very low voltage operation and/or very low power consumption.